Exhaust gas sensors are often disposed in the exhaust passages of internal combustion engines for detecting a physical quantity as to an exhaust gas component state, such as an exhaust gas component concentration, for the purpose of controlling the operation of the internal combustion engine or monitoring the status of an exhaust gas purifying system. Specifically, the exhaust gas sensor is disposed at a certain location in the exhaust gas passage and has an element sensitive to an exhaust gas component state to be detected, the element being positioned for contact with an exhaust gas flowing through the exhaust passage. For example, an air-fuel ratio sensor such as an O2 sensor or the like is disposed as an exhaust gas sensor upstream or downstream of an exhaust gas purifying catalyst disposed in the exhaust passage for the purpose of controlling the air-fuel ratio of the internal combustion engine in order to keep well the purifying ability of the catalyst.
Some air-fuel ratio sensors have a built-in heater for heating the active element thereof for increasing the temperature of the element and activating the element to enable the element to perform its essential functions and also removing foreign matter deposited on the element. For example, an air-fuel ratio sensor such as an O2 sensor or the like usually has an electric heater for heating the active element thereof. After the internal combustion engine has started to operate, the electric heater is energized to increase the temperature of the active element of the O2 sensor to activate the active element and keep the active element active.
As shown in FIG. 3 of the accompanying drawings, the O2 sensor produces an output voltage Vout which changes with a large gradient with respect to a change in the air-fuel ratio of an exhaust gas, i.e., which is highly sensitive to a change in the air-fuel ratio, only in a small range Δ (near a stoichiometric air-fuel ratio) of values of the air-fuel ratio that is represented by an oxygen concentration in the exhaust gas to which the active element is sensitive. A change in the output voltage Vout of the O2 sensor, i.e., a gradient of the output voltage Vout with respect to the air-fuel ratio, is smaller in air-fuel ratio ranges that are richer and leaner than the highly sensitive range Δ. The output characteristics of the O2 sensor, i.e., the gradient of the highly sensitive range Δ, etc., vary depending on the temperature of the active element. When the air-fuel ratio is to be controlled using the output voltage from the O2 sensor, therefore, it is desirable to keep the output characteristics of the O2 sensor in a desired range as much as possible and hence to keep the temperature of the active element of the O2 sensor in a desired temperature range as stably as possible for better air-fuel ratio control.
Not only O2 sensors but also many exhaust gas sensors have their output characteristics affected by the temperature of the active element. If the internal combustion engine is to be controlled using the output signal from the O2 sensor, then it is preferable to keep the temperature of the active element of the exhaust gas sensor in a desired temperature range as stably as possible for better engine control. When the active element of the exhaust gas sensor is heated to clean the active element, it is also preferable to keep the temperature of the active element of the exhaust gas sensor in a desired temperature range for a better cleaning effect.
As disclosed in Japanese laid-open patent publication No. 2000-304721 by the applicant of the present application, it is known to estimate the temperature of the active element of an exhaust gas sensor (an air-fuel ratio sensor in the publication) and control the energization of a heater (an electric heater) based on the estimated temperature for thereby keep the temperature of the active element in a desired temperature range to obtain appropriate output characteristics from the exhaust gas sensor. According to the disclosed arrangement, the resistance of the heater is recognized from detected values of a current flowing through the heater and a voltage applied across the heater, and the temperature of the active element is estimated based on the detected resistance of the heater.
According to the disclosure of the above publication, however, since a duty cycle which determines the electric power to be supplied to the heater is uniquely determined by a table from an estimated value of the temperature of the active element of the exhaust gas sensor, it is difficult to control the temperature of the active element of the exhaust gas sensor stably at a desired temperature because of a change in the temperature of the exhaust gas, an ambient air temperature, etc. Furthermore, if the temperature of the active element of the exhaust gas sensor is low, then in order to keep the duty cycle of the heater at a certain maximum level, the heater tends to consume more electric power than required and the actual temperatures of the heater and the active element are liable to be excessively high, causing damage to the heater and the active element.
The present invention has been made in view of the above background. It is an object of the present invention to provide an apparatus for and a method of controlling the temperature of the active element of an exhaust gas sensor stably at a desired temperature. Another object of the present invention is to provide a recording medium storing a temperature control program for controlling the temperature of the active element of an exhaust gas sensor stably at a desired temperature.